This invention relates to fiber optic sensors, and more particularly to tactile sensors, which are used to characterize an object by measuring the spatial distribution of the contact forces involved in gripping the object.
Sensors enable a robot to respond to changes in its environment. This adaptibility, particularly with respect to the tactile sensing required in manipulation activities, must be developed for future generations of intelligent machines if those machines are to realize the full potential made possible by advances in the state of the art of computing power, artificial intelligence, and mechanical, structural, and control systems.
An ideal tactile sensor should provide both a qualitative image, representative of human tactility, and the quantitative force information which is needed to generate control commands for mechanical systems. Electromagnetic and electrostatic interference is a principal concern because of the complexity of the wiring, addressing, and signal processing aspects of a sensing system that must be placed in close proximity to the tactile transducer. Some combination of preamplification, analog to digital conversion, and, for arrays of transducers, matrix addressing, clocking, and data bus transmission must be configured close to the sensing surface to reduce these interference effects.
A variety of sensing technologies have been applied to this task in the prior art, including conductive elastomers, ferroelectric polymers, silicon strain gauges, magnetostriction, capacitance, and optoelectronics, including fiber optic sensing techniques. Among these different approaches, fiber optics is a particularly attractive technology for tactile sensors because of the reduction in electromagnetic and electrostatic interference which occurs when system data is transmitted optically rather than electrically. The most common applications of fiber optic sensing in the prior art use a source fiber to deliver light to a reflecting or scattering surface through a transparent elastomer or across a gap modulated by pressure on a compliant support structure. A receptor fiber picks up the returned light, which is then converted remotely by a photodiode into an analog electrical signal. The direct response of such a system is usually very nonlinear, but can be adequately modeled and calibrated. Where the fiber itself is employed as the sensing element, forming an array of sensing sites and reading them results in a complexity of conductors, albeit conductors made of glass rather than metal, that must be addressed. The signals from this array must be processed in much the same way as more conventional matrix or array transducer schemes.
Fiber optic sensors can be grouped into three basic categories according to their mode of operation:
(1) the modulation of phase or polarization in single mode fibers, PA0 (2) microbending-induced amplitude modulation in multimode fibers, and PA0 (3) other mechanical amplitude modulators, such as relative displacement and Schlieren grating devices.
Single mode fibers have been used to detect sound, rotation, or other parameters by injecting light at one end of the fiber and detecting it at the other. The physical principle involves a comparison of the phase shift, or interference, between light in the sensor fiber and in the reference fiber. This detection technique, however, requires a reference fiber, optical alignment and coupling are extremely sensitive, and the required electronic circuitry is complex. The resulting signal corresponds only to the total integrated effect along the length of the sensing fiber. Whereas the theoretical sensitivity of the single mode approach is high, laboratory systems have been known to lose as much as three orders of magnitude in sensitivity when installed in operational environments.
Multimode fiber sensing systems benefit from fabrication uniformity and well developed connector technology, packaging, and characterization equipment. The operation of multimode sensors depends upon the reduction in the amount of light energy which is transmitted through the fiber when a continuous single fiber is deformed. This decrease occurs because light in an unperturbed fiber is waveguided through the core of the fiber by total internal reflection in a spectrum of modes determined by the core diameter and the ratio of the optical indices of the core and the surrounding cladding. When the fiber is bent and the cylindrical symmetry is destroyed, light scatters into other modes of propagation, some of which include radiation out of the fiber. Such sensors are termed amplitude sensors. As with monomode fibers, however, the simplest systems do not provide information about the location along the fiber at which a deformation occurs.
Consequently, a need has arisen in the art of tactile sensors for a fiber optic sensor which requires only a relatively simple arrangement of sensing and data transmitting elements while being capable of accurately and reproducibly sensing applied force over a two dimensional area without undue interference from electromagnetic or electrostatic interference.